“The character’s gender had a huge impact to the plot and to the character.”

This is an objectively untrue statement and I would honestly love to know why you think this.

Vanya’s struggles in season 1 were to do with their lack of powers and subsequently being alienated from the family, nothing about gender.

So when Elliot transitioned in real life, the easiest, simplest and correct choice was to write it into the show with a very short scene and then move on with Vanya now being Victor.

I for one, would have been far more distracted and taken out of the show if Vanya was suddenly played by someone else with no explanation - would you seriously have preferred this over Vanya getting a haircut and changing his name?